1. Field of the Invention
The present invention relates to a stepping motor and a timepiece.
2. Description of Related Art
A stepping motor including a rotor, a stator, and a coil is used to drive various devices.
The stepping motor can rotate the rotor by a predetermined step angle through application of voltage to the coil. Such a stepping motor is widely applied to devices, such as a timepiece including hands (a second hand, a minute hand, and an hour hand), that need to accurately operate by a predetermined amount.
In the invention described in WO99/064937, the stator has a pair of outer notches on a line orthogonal to a linear portion of a coil core provided with a coil, such that the outer notches are disposed on the opposite sides of a rotor accommodating hole for accommodating the rotor. This configuration defines two positions of saturated magnetic fluxes.
The stator also has a pair of inner notches on the inner periphery around the rotor accommodating hole on the substantially opposite sides. The inner notches serve as position determiners for determining positions (stably stationary positions) where the rotor stably stops.
Unfortunately, in such a conventional stepping motor, for example, if the inner notches are provided around the rotor accommodating hole so as to stop the rotor at an angle of approximately 45° from the positions of saturated magnetic fluxes, the stator has narrow portions adjacent to the positions of saturated magnetic fluxes.
The magnetic fluxes generated from the coil core flow to the narrow portions before reaching the positions of saturated magnetic fluxes in the stator, so that the magnetic fluxes are readily saturated at the narrow portions. This configuration causes energy loss.
In general, a motor should be further miniaturized to be installed in a small electronic device such as a wristwatch. Unfortunately, the miniaturization of the motor further narrows the narrow portions to accelerate saturation of the magnetic fluxes at the narrow portions. The energy loss accordingly increases.